Plasminogen activator inhibitor - 1(PAI-1) is the primary physiologic inhibitor of tissue plasminogen activator (tPA) and also inhibits urokinase (UK). Studies suggest that PAI-1 is the major regulator of tPA induced fibrinolysis and may also be involved in other processes including matrix degradation and tissue remodeling. Elevated levels of plasma PAI-1 are associated with thrombosis in man. Controversy exists whether liver or vascular endothelium is the major source of plasma PAI-1 and studies suggest that PAI-1 synthesis is regulated differently in these two tissues. We have preliminary evidence in rat that PAI-1 mRNA is expressed primarily in the hepatic endothelium and not the hepatocyte. Studies outlined in this proposal will compare PAI-1 mRNA and protein synthesis in freshly isolated rat hepatic fractions and aortic endothelium and in cultured cells from these same tissues. Cultures will be treated with modulators known to affect PAI-1 expression in endothelium or hepatocyte/hepatoma cultures to evaluate differential regulation. PAI-1 expression in human endothelium is modulated by a number of factors including cAMP elevating compounds and phorbol esters. We had previously shown that the class I heparin binding growth factor, endothelial cell growth factor (HBGF-1), decreased PAI-1 expression in human endothelial cell culture. Recent studies in my laboratory have shown that cAMP elevation and the phorbol ester PMA modulate the HBGF-1 effect on PAI-1. Studies are outlined to further investigate cellular pathways involved in this growth factor modulation of PAI-1 expression. Human PAI-1 expression is regulated, at least in part, at the level of gene transcription. We have evidence that it is also modulated at the level of mRNA stability. An "AU" rich sequence in the 3' untranslated region of PAI-1 mRNA may be involved in these processes. Studies are outlined in this proposal to investigate mRNA sequences and RNA binding protein that may be involved in regulating PAI-1 mRNA stability.